Multiple tunable central cathodes on a paddle for increased medial-lateral and rostral-caudal flexibility via current steering

ABSTRACT

A neurostimulation paddle lead, method of neurostimulation, and neurostimulation system are provided. The neurostimulation paddle lead carries a plurality of electrodes comprising at least four columns of electrodes having a spacing between two inner electrode columns less than a spacing between the inner electrode columns and adjacent outer electrode columns. The inner electrode columns may also be longitudinally offset from the outer electrode columns. The methods and neurostimulation systems steer current between the electrodes to modify a medial-lateral electrical field created adjacent spinal cord tissue.

CLAIM OF PRIORITY

This application is a continuation of U.S. application Ser. No.12/204,114, filed Sep. 4, 2008, which is hereby incorporated byreference in its entirety.

RELATED APPLICATIONS

This application is filed concurrently with U.S. patent application Ser.No. 12/204,094, now U.S. Pat. No. 7,987,000, U.S. patent applicationSer. No. 12/204,136, now U.S. Pat. No. 8,437,857, U.S. patentapplication Ser. No. 12/204,154, now U.S. Pat. No. 8,442,655 and U.S.patent application Ser. No. 12/204,170, now U.S. Pat. No. 9,504,818, thedisclosures of which are expressly incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to tissue stimulation systems, and moreparticularly, to systems and methods for electrically stimulating spinalcord tissue.

BACKGROUND OF THE INVENTION

Spinal cord stimulation (SCS) is a well-accepted clinical method forreducing pain in certain populations of patients. During SCS, the spinalcord, spinal nerve roots, or other nerve bundles are electricallystimulated using one or more neurostimulation leads implanted adjacentthe spinal cord. While the pain-reducing effect of SCS is not wellunderstood, it has been observed that the application of electricalenergy to particular regions of the spinal cord induces paresthesia(i.e., a subjective sensation of numbness or tingling) that replaces thepain signals sensed by the patient in the afflicted body regionsassociated with the stimulated spinal regions. Thus, the paresthesiaappears to mask the transmission of chronic pain sensations from theafflicted body regions to the brain.

In a typical procedure, one or more stimulation leads are introducedthrough the patient's back into the epidural space under fluoroscopy,such that the electrodes carried by the leads are arranged in a desiredpattern and spacing to create an electrode array. The specific procedureused to implant the stimulation leads will ultimately depend on the typeof stimulation leads used. Currently, there are two types ofcommercially available stimulation leads: a percutaneous lead and asurgical lead.

A percutaneous lead comprises a cylindrical body with ring electrodes,and can be introduced into contact with the affected spinal tissuethrough a Touhy-like needle, which passes through the skin, between thedesired vertebrae, and into the epidural space above the dura layer. Forunilateral pain, a percutaneous lead is placed on the correspondinglateral side of the spinal cord. For bilateral pain, a percutaneous leadis placed down the midline of the spinal cord, or two percutaneous leadsare placed down the respective sides of the midline. A surgical lead hasa paddle on which multiple electrodes are arranged typically inindependent columns, and is introduced into contact with the affectedspinal tissue using a surgical procedure, and specifically, alaminectomy, which involves removal of the laminar vertebral tissue toallow both access to the dura layer and positioning of the lead.

Stimulation energy may be delivered to the electrodes of the leadsduring and after the placement process in order to verify that the leadsare stimulating the target neural tissue. Stimulation energy is alsodelivered to the electrodes at this time to formulate the most effectiveset of stimulus parameters, which include the electrodes that aresourcing (anodes) or returning (cathodes) the stimulation pulses at anygiven time, as well as the magnitude and duration of the stimulationpulses. During the foregoing procedure, an external trialneurostimulator may be used to convey the stimulation pulses to thelead(s), while the patient provides verbal feedback regarding thepresence of paresthesia over the pain area. The stimulus parameter setwill typically be one that provides stimulation energy to all of thetarget tissue that must be stimulated in order to provide thetherapeutic benefit (e.g., pain relief), yet minimizes the volume ofnon-target tissue that is stimulated, which may correspond to unwantedor uncomfortable paresthesia. Thus, neurostimulation leads are typicallyimplanted with the understanding that the stimulus parameter set willrequire fewer than all of the electrodes on the leads to achieve thedesired paresthesia.

After the lead(s) are placed at the target area of the spinal cord, thelead(s) are anchored in place, and the proximal ends of the lead(s), oralternatively lead extensions, are passed through a tunnel leading to asubcutaneous pocket (typically made in the patient's abdominal area)where a neurostimulator is implanted. The lead(s) are connected to theneurostimulator, which is programmed with the stimulation parameterset(s) previously determined during the initial placement of thelead(s). The neurostimulator may be operated to test the effect ofstimulation and, if necessary, adjust the programmed set(s) ofstimulation parameters for optimal pain relief based on verbal feedbackfrom the patient. Based on this feedback, the lead position(s) may alsobe adjusted and re-anchored if necessary. Any incisions are then closedto fully implant the system.

The efficacy of SCS is related to the ability to stimulate the spinalcord tissue corresponding to evoked paresthesia in the region of thebody where the patient experiences pain. Thus, the working clinicalparadigm is that achievement of an effective result from SCS depends onthe neurostimulation lead or leads being placed in a location (bothlongitudinal and lateral) relative to the spinal tissue such that theelectrical stimulation will induce paresthesia located in approximatelythe same place in the patient's body as the pain (i.e., the target oftreatment). If a lead is not correctly positioned, it is possible thatthe patient will receive little or no benefit from an implanted SCSsystem. Thus, correct lead placement can mean the difference betweeneffective and ineffective pain therapy.

Even after successful placement of the leads in the operating room (withcorresponding test stimulation), the SCS system typically requireselectrical fine-tuning post-operatively, and often it is difficult totarget all pain areas, with some areas (e.g., the lower back) beingparticularly difficult to target. In particular, lead migration mayrelocate the paresthesia away from the pain site, resulting in thetarget neural tissue no longer being appropriately stimulated and thepatient no longer realizing the full intended therapeutic benefit. Withelectrode programmability, the stimulation area can often be moved backto the effective pain site without having to reoperate on the patient inorder to reposition the lead. For example, some SCS systems use changesin electrode polarity or incremental electrical current shifts in thecathodes and anodes to tune the location of paresthesia.

To produce the feeling of paresthesia without inducing discomfort orinvoluntary motor movements within the patient, it is often desirable topreferentially stimulate nerve fibers in the dorsal column (DC nervefibers), which primarily include sensory nerve fibers, over nerve fibersin the dorsal roots (DR nerve fibers), which include both sensory nervefibers and motor reflex nerve fibers. In order to stimulate the DC nervefibers, while guarding against the stimulation of the DR nerve fibers,SCS systems may activate anodes that flank a single cathode in amedial-lateral electrical field, with the single cathode providing thestimulation energy for the DC fibers, while the flanking anodes guardingagainst the over-stimulation of the DR fibers, as illustrated in FIG. 1.

While change in the relative anode strengths will yield some tunabilitywith a multiple source system, the electrical field is “tethered” to thesingle cathode, and so has limited flexibility in medial-lateral tuning.In fact, some hypotheses would suggest that “lower-back fibers” areoff-midline, and thus a single cathode located over the center of thespinal cord may not be the optimum position for the cathode.

Also, the fixed spacing between the anodes and the cathode in a 3-columnmedial-lateral electrode arrangement is limiting, because the spacingwould ideally be optimized to the distance from the electrodes to thespinal cord (due, e.g., to cerebral spinal fluid thickness (dCSF)), andthat is a parameter with substantial variability between patients and atdifferent vertebral levels within a patient. That is, in the case of ahigh dCSF, the spinal cord tissue will be relatively far away from theelectrodes, and, therefore, it is desirable to increase the spacingbetween the anodes and cathode to lower the stimulation threshold byreducing the shunting of current, thus preventing excessive amplitudes.In the case of a low dCSF, the spinal cord tissue will be relativelyclose to the electrodes, and thus, current shunting (i.e., decay offield strength) is not as critical. In this case, it is desirable toincrease the tunability of the stimulation by decreasing the spacingbetween the anodes and cathode. However, because the physical spacingbetween the anodes and cathode is fixed, and prior art SCS systems donot have the capability of electrically adjusting the spacing betweenthe flanking anodes and the single cathode, variations in the dCSFcannot be suitably accounted for in prior art SCS systems. In addition,in a prior art medial-lateral arrangement, the electrodes are uniformlyspaced and rostral-caudally aligned with each, which may not be theoptimum arrangement.

There, thus, remains a need for an improved SCS system with improvedtargeting capability using a medial-lateral electrode arrangement.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present inventions, aneurostimulation paddle lead comprises at least one elongated lead body,a plurality of terminals carried by the proximal end of the lead body,and a paddle-shaped membrane disposed on the distal end of the leadbody(ies). In one embodiment, the paddle-shaped membrane is sized andshaped to be disposed within the epidural space of a patient. The paddlelead further comprises a plurality of electrodes arranged on an exteriorsurface of the paddle-shaped membrane in electrical communication withthe respective terminals.

The plurality of electrodes comprises at least four electrodes extendingalong the paddle-shaped membrane in a direction transverse to thelongitudinal axis of the lead body. These electrodes include first andsecond inner electrodes immediately adjacent to each other, a firstouter electrode immediately adjacent to the first inner electrode, and asecond outer electrode immediately adjacent to the second innerelectrode. A first transverse spacing between the first and second innerelectrodes is different from each of a second transverse spacing betweenthe first inner electrode and the first outer electrode, and a thirdtransverse spacing between the second inner electrode and the secondouter electrode. For example, the first transverse spacing can be lessthan each of the second and third transverse spacings. In oneembodiment, the first transverse spacing is within the range of 1.5-4.0mm, and each of the second and third transverse spacings, which may beequal to each other, is within the range of 1.5-4.0 mm. In anotherembodiment, the transverse spacing between the first and second outerelectrodes is within the range of 4.5-11.0 mm. In another embodiment,the first and second inner electrodes are offset from the first andsecond outer electrodes along the longitudinal axis.

In accordance with a second aspect of the present inventions, anotherneurostimulation paddle lead is similar to the previously describedpaddle lead, with the exception that the plurality of electrodescomprises at least four columns of electrodes. Each electrode columnextends along the paddle-shaped membrane in a longitudinal direction,with the electrode columns having first and second inner electrodecolumns immediately adjacent to each other, a first outer electrodecolumn immediately adjacent to the first inner electrode column, and asecond outer electrode column immediately adjacent to the second innerelectrode column. The first transverse spacing between the first andsecond inner electrode columns is different for each of a secondtransverse spacing between the first inner electrode column and thefirst outer electrode column, and a third transverse spacing between thesecond inner electrode column and the second outer electrode column. Forexample, the first transverse spacing can be less than each of thesecond and third transverse spacings. In one embodiment, the firsttransverse spacing is within the range of 1.5-4.0 mm, and each of thesecond and third transverse spacings, which may be equal to each other,is within the range of 1.5-4.0 mm. In another embodiment, the transversespacing between the first and second outer electrode columns is withinthe range of 4.5-11.0 mm. In another embodiment, the first and secondinner electrode columns are offset from the first and second outerelectrode columns along the longitudinal axis.

In accordance with a third aspect of the present inventions, a method ofproviding therapy to a patient comprises disposing at least fourelectrodes adjacent spinal cord tissue of the patient (e.g., in theepidural space of the patient) in a medial-lateral electrodearrangement. The electrodes include first and second inner electrodesimmediately adjacent to each other, a first outer electrode immediatelyadjacent to the first inner electrode, and a second outer electrodeimmediately adjacent to the second inner electrode. A firstmedial-lateral spacing between the first and second inner electrodes isdifferent from each of a second medial-lateral spacing between the firstinner electrode and the first outer electrode, and a thirdmedial-lateral spacing between the second inner electrode and the secondouter electrode. For example, the first medial-lateral spacing can beless than each of the second and third medial-lateral spacings.

In one method, the first medial-lateral spacing is within the range of1.5-4.0 mm, and each of the second and third medial-lateral spacings,which may be equal to each other, is within the range of 1.5-4.0 mm. Inanother method, the medial-lateral spacing between the first and secondouter electrodes is within the range of 4.5-11.0 mm. In another method,the first and second inner electrodes are rostralcaudally offset fromthe first and second outer electrodes.

The method further comprises configuring at least one of the first andsecond inner electrodes as a cathode, and at least one of the first andsecond outer electrodes as an anode. In one method, the both of thefirst and second inner electrodes are configured as cathodes, and bothof the outer electrodes are configured as anodes. The method furthercomprises conveying electrical energy between the cathode(s) and theanode(s) that creates a medial-lateral electrical field that stimulatesthe spinal cord tissue.

In accordance with a fourth aspect of the present invention, aneurostimulation paddle lead comprises at least one elongated lead body,a plurality of terminals carried by the proximal end of the lead body,and a paddle-shaped membrane disposed on the distal end of the leadbody(ies). In one embodiment, the paddle-shaped membrane is sized andshaped to be disposed within the epidural space of a patient. The paddlelead further comprises a plurality of electrodes arranged on an exteriorsurface of the paddle-shaped membrane in electrical communication withthe respective terminals.

The plurality of electrodes comprises at least four columns ofelectrodes, with each column extending along the paddle-shaped membranein a longitudinal direction. The electrode columns include at least twoinner electrode columns and outer electrode columns flanking the innerelectrode column(s). At least one of the inner electrode columns isoffset from the outer electrode columns in the longitudinal direction.In one embodiment, at least one electrode in each of the inner electrodecolumns is equi-distant between two immediately adjacent electrodes ineach of the outer electrode columns.

In one embodiment, the inner electrode columns comprises first andsecond immediately adjacent inner electrode columns, a first one of theouter electrode columns is immediately adjacent the first innerelectrode column, and a second one of the outer electrode columns isimmediately adjacent the second inner electrode column. In this case, afirst transverse spacing between the first and second inner electrodecolumns may be different from each of a second transverse spacingbetween the first inner electrode column and the first outer electrodecolumn, and a third transverse spacing between the second innerelectrode column and the second outer electrode column. For example, thefirst transverse spacing can be less than each of the second and thirdtransverse spacings. In one embodiment, the first transverse spacing iswithin the range of 1.5-4.0 mm, and each of the second and thirdtransverse spacings, which may be equal to each other, is within therange of 1.5-4.0 mm. In another embodiment, the transverse spacingbetween the first and second outer electrode columns is within the rangeof 4.5-11.0 mm. In another embodiment, the first and second innerelectrode columns are offset from the first and second outer electrodecolumns along the longitudinal axis.

In accordance with a fifth aspect of the present inventions, a method ofproviding therapy to a patient comprises disposing at least four columnsof electrodes adjacent spinal cord tissue of the patient (e.g., withinthe epidural space of the patient), with the electrode column(s) have atleast two inner electrode columns and outer electrode columns flankingthe inner electrode column(s).

The method further comprises configuring at least one electrode in theinner electrode column as a cathode, and at least two immediatelyadjacent electrodes in each of at least one of the outer electrodecolumns as anodes, with the anodes being positioned both rostrally andcaudally relative to the cathode(s). The method further comprisesconveying electrical energy between the cathode(s) and the anodes tocreate a medial-lateral electrical field that stimulates the spinal cordtissue.

In one method, at least one electrode in each of the two inner electrodecolumns is configured as a cathode. In this case, the cathodes may beimmediately adjacent to each other, and electrodes in both the outerelectrode columns can be configured as anodes. In another method, atleast one electrode in only one of the two inner electrode columns isconfigured as a cathode. In this case, electrodes in both the outercolumns(s) may be configured as anodes, or electrodes in only the outerelectrode column immediately adjacent the inner electrode column may beconfigured as anodes, or electrodes in only the outer electrode columnthat is not immediately adjacent the inner electrode column may beconfigured as anodes.

In still another method, electrodes in first and second inner electrodecolumns that are immediately adjacent to each other are configured ascathodes, and anodes in a first outer electrode column immediatelyadjacent to the first inner electrode column, and electrodes in a secondouter electrode column immediately adjacent to the second innerelectrode column are configured as anodes. In this case, a firsttransverse spacing between the first and second inner electrode columnsis different from each of a second transverse spacing between the firstinner electrode column and the first outer electrode column, and a thirdtransverse spacing between the second inner electrode column and thesecond outer electrode column. For example, the first transverse spacingcan be less than each of the second and third transverse spacings.

In accordance with a sixth aspect of the present inventions, a method ofproviding therapy to a patient comprises disposing at least fourelectrodes adjacent the spinal cord tissue of the patient (e.g., in theepidural space) in a medial-lateral electrode arrangement. Theelectrodes include two inner electrodes and two outer electrodesflanking the two inner electrodes. The method further comprisesconfiguring the inner electrodes as cathodes, and the outer electrodesas anodes, and conveying electrical energy between the cathodes and theanodes to create a medial-lateral electrical field that stimulates thespinal cord tissue; for example, by stimulating the dorsal column fiberswithout stimulating the dorsal root fibers.

The method further comprises incrementally shifting cathodic currentbetween the cathodes (e.g., in increments equal to or less than 10percent) to modify the medial-lateral electrical field. For example, thecathodic current may be incrementally shifted between the cathodes tospatially shift the medial-lateral electrical field transverselyrelative to dorsal column fibers of the spinal cord tissue. One methodfurther comprises incrementally shifting anodic current between theanodes to modify the medial-lateral electrical field.

In accordance with a seventh aspect of the present inventions, aneurostimulation system for providing therapy to a patient comprises aneurostimulation paddle lead carrying a plurality of electrodescomprising at least four electrodes extending in a direction transverseto a longitudinal axis of the paddle lead. In one embodiment, the paddlelead is sized to be implanted within an epidural space above spinal cordtissue of the patient. The electrodes include two inner electrodes andtwo outer electrodes flanking the two inner electrodes.

The neurostimulation system further comprises a neurostimulator forconfiguring the inner electrodes as cathodes and the outer electrodes asanodes, and for conveying electrical energy between the cathodes and theanodes to create an electrical field that stimulates tissue of thepatient; for example, by stimulating the dorsal column fibers withoutstimulating the dorsal root fibers. The neurostimulator is furtherconfigured for incrementally shifting cathodic current between thecathodes (e.g., in increments equal to or less than 10 percent) tomodify the electrical field. For example, the neurostimulator may beconfigured for incrementally shifting the cathodic current between thecathodes to spatially shift the electrical field transversely relativeto dorsal column fibers of the spinal cord tissue. In one embodiment,the neurostimulator is configured for incrementally shifting anodiccurrent between the anodes to modify the electrical field.

In accordance with an eighth aspect of the present inventions, a methodof providing therapy to a patient comprises disposing at least fourcolumns of electrodes adjacent spinal cord tissue of the patient (e.g.,within the epidural space). The electrode columns include two innerelectrode columns and two outer electrode columns flanking the two innerelectrode columns. The method further comprises configuring at least twoelectrodes of the inner electrode columns as cathodes, and at least oneelectrode of each of the outer electrode columns as anodes, conveyingelectrical energy between the cathodes and the anodes to create amedial-lateral electrical field that stimulates the spinal cord tissue(for example, by stimulating the dorsal column fibers withoutstimulating the dorsal root fibers), and incrementally shifting cathodiccurrent between the cathodes (e.g., in increments equal to or less than10 percent) to modify the medial-lateral electrical field.

In one method, the cathodes are rostral-caudally aligned relative toeach other (i.e., the cathodes are at the same rostral-caudal level),such that incremental shifting of the cathodic current between thecathodes spatially shifts the medial-lateral electrical fieldtransversely relative to dorsal column fibers of the spinal cord tissue.In another method, the cathodes are rostral-caudally offset from eachother, such that incremental shifting of the cathodic current betweenthe cathodes rostral-caudally expands the medial-lateral electricalfield.

In accordance with a ninth aspect of the present inventions, aneurostimulation system for providing therapy to a patient comprises aneurostimulation paddle lead carrying a plurality of electrodescomprising at least four columns of electrodes extending along alongitudinal axis of the paddle lead. The electrode columns include twoinner electrode columns and two outer electrode columns flanking the twoinner electrode columns. In one embodiment, the paddle lead is sized tobe implanted within an epidural space above spinal cord tissue of thepatient.

The neurostimulation system further comprises a neurostimulator forconfiguring at least two electrodes of the inner electrode columns ascathodes, and at least one electrode of each of the outer electrodecolumns as anodes, conveying electrical energy between the cathodes andthe anodes to create an electrical field that stimulates tissue of thepatient (for example, by stimulating the dorsal column fibers withoutstimulating the dorsal root fibers), and incrementally shifting cathodiccurrent between the cathodes (e.g., in increments equal to or less than10 percent) to modify the electrical field. In one embodiment, thecathodes are longitudinally aligned relative to each other (i.e., thecathodes are at the same longitudinal level), such that incrementalshifting of the cathodic current between the cathodes spatially shiftsthe electrical field transversely relative to the longitudinal axis. Inanother embodiment, the cathodes are longitudinally offset from eachother, such that incremental shifting of the cathodic current betweenthe cathodes expands the electrical field along the longitudinal axis.

In accordance with a tenth aspect of the present inventions, a method ofproviding therapy to a patient comprises disposing at least fourelectrodes adjacent spinal cord tissue of the patient (e.g., in theepidural space) in a medial-lateral electrode arrangement. Theelectrodes include two inner electrodes and two outer electrodesflanking the two inner electrodes. In one method, the transverse pacingbetween the second one of the inner electrodes and the first one of theinner electrodes is less than the transverse spacing between a closestone of the outer electrodes and the second one of the inner electrodes.

The method further comprises configuring a first one of the innerelectrodes as a cathode, a second one of the inner electrodes as one ofa cathode and an anode, and the outer electrodes as anodes, andconveying electrical energy between the cathodes and the anodes tocreate a medial-lateral electrical field that stimulates the spinal cordtissue; for example, by stimulating the dorsal column fibers withoutstimulating the dorsal root fibers. The method further comprisesreconfiguring the second one of the inner electrodes as the other of thecathode and the anode, and reconveying electrical energy between thecathodes and the anodes to create a medial-lateral electrical field thatstimulates the spinal cord tissue.

One method further comprises incrementally shifting (e.g., in incrementsequal to or less than 10 percent) cathodic current between the cathodesand/or anodic current between the anodes to modify the medial-lateralelectrical field. For example, the cathodic current may be incrementallyshifted between the cathodes to spatially shift the medial-lateralelectrical field transversely relative to dorsal column fibers of thespinal cord tissue.

In accordance with an eleventh aspect of the present inventions, aneurostimulation system for providing therapy to a patient comprises aneurostimulation paddle lead carrying a plurality of electrodescomprising at least four electrodes extending in a direction transverseto a longitudinal axis of the paddle lead. The electrodes include twoinner electrodes and two outer electrodes flanking the two innerelectrodes. In one embodiment, the paddle lead is sized to be implantedwithin an epidural space above spinal cord tissue of the patient. Inanother embodiment, the spacing between the second one of the innerelectrodes and the first one of the inner electrodes is less than thespacing between a closest one of the outer electrodes and the second oneof the inner electrodes.

The neurostimulation system further comprises a neurostimulator forconfiguring a first one of the inner electrodes as a cathode, a secondone of the inner electrodes as one of a cathode and an anode, and theouter electrodes as anodes, conveying electrical energy between thecathodes and the anodes to create an electrical field that stimulatestissue of the patient (for example, by stimulating the dorsal columnfibers without stimulating the dorsal root fibers), reconfiguring thesecond one of the inner electrodes as the other of the cathode and theanode, and reconveying electrical energy between the cathodes and theanodes to create an electrical field that stimulates the tissue. In oneembodiment, the neurostimulator is further configured for incrementallyshifting cathodic current between the cathodes and/or anodic currentbetween the anodes (e.g., in increments equal to or less than 10percent) to modify the electrical field. For example, the cathodiccurrent may be incrementally shifted between the cathodes to spatiallyshift the medial-lateral electrical field transversely relative todorsal column fibers of the spinal cord tissue.

In accordance with a twelfth aspect of the present inventions, a methodof providing therapy to a patient comprises disposing at least fourelectrodes adjacent spinal cord tissue of the patient (e.g., in theepidural space) in a medial-lateral electrode arrangement. Theelectrodes include two inner electrodes and two outer electrodesflanking the two inner electrodes.

The method further comprises conveying electrical energy between theelectrodes to create a medial-lateral electrical field having a locus onone lateral side of the midline of the spinal cord tissue (e.g., byconfiguring only a first one of the inner electrodes as a cathode orconfiguring a first one of the inner electrodes to have more cathodiccurrent than a second one of the inner electrodes), and conveyingelectrical energy between the electrodes to create a medial-lateralelectrical field having a locus on the other lateral side of the midlineof the spinal cord tissue (e.g., by configuring only a second one of theinner electrodes as a cathode or by configuring the second one of theinner electrodes to have more cathodic current than the first one of theinner electrodes). In one method, the medial-lateral electrical fieldsstimulate the dorsal column fibers without stimulating dorsal rootfibers within the spinal cord tissue.

In accordance with a thirteenth aspect of the present invention, aneurostimulation system for providing therapy to a patient comprises aneurostimulation paddle lead configured for being implanted within anepidural space above spinal cord tissue of the patient. Theneurostimulation paddle lead carries a plurality of electrodescomprising at least four electrodes extending in a direction transverseto a longitudinal axis of the paddle lead. The electrodes include twoinner electrodes and two outer electrodes flanking the two innerelectrodes.

The neurostimulator is configured for conveying electrical energybetween the electrodes to create a medial-lateral electrical fieldhaving a locus on one lateral side of the midline of the spinal cordtissue (e.g., by configuring only a first one of the inner electrodes asa cathode or configuring a first one of the inner electrodes to havemore cathodic current than a second one of the inner electrodes), andconveying electrical energy between the electrodes to create amedial-lateral electrical field having a locus on the other lateral sideof the midline of the spinal cord tissue (e.g., by configuring only asecond one of the inner electrodes as a cathode or by configuring thesecond one of the inner electrodes to have more cathodic current thanthe first one of the inner electrodes). In one embodiment, theneurostimulator is configured for conveying the electrical energybetween the cathodes and the anodes to create an electrical field thatstimulates the dorsal column fibers without stimulating dorsal rootfibers within the spinal cord tissue.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.

Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is a cross-sectional view of a spinal cord and a prior artelectrode arrangement for creating a medial-lateral electrical fieldthat stimulates the spinal cord;

FIG. 2 is plan view of one embodiment of a spinal cord stimulation (SCS)system arranged in accordance with the present inventions;

FIG. 3 is a profile view of an implantable pulse generator (IPG) and oneembodiment of a neurostimulation paddle lead used in the SCS system ofFIG. 2;

FIG. 4 is a profile view of an implantable pulse generator (IPG) andanother embodiment of neurostimulation paddle lead used in the SCSsystem of FIG. 2;

FIG. 5 is a profile view of an implantable pulse generator (IPG) andstill another embodiment of a neurostimulation paddle lead used in theSCS system of FIG. 2;

FIG. 6 is a block diagram of the internal components of an IPG used inthe SCS system of FIG. 2;

FIG. 7 is a plan view of the implantable components of the SCS system ofFIG. 2 in use with a patient;

FIG. 8 is a coronal view of a cathode-anode electrode arrangement thatcan create a medial-lateral electrical field to stimulate spinal cordtissue using the neurostimulation paddle lead of FIG. 5;

FIG. 9 is a coronal view of a cathode-anode electrode arrangement thatcan create a medial-lateral electrical field to stimulate spinal cordtissue using the neurostimulation paddle lead of FIG. 3;

FIGS. 10a-10d are coronal views of different cathode-anode electrodearrangements that can create different medial-lateral electrical fieldsto stimulate spinal cord tissue using the neurostimulation paddle leadof FIG. 5;

FIGS. 11a-11e are cross-sectional views of a cathode-anode electrodearrangement that can transversely steer a medial-lateral electricalfield relative to spinal cord tissue using any of the neurostimulationpaddle leads of FIGS. 3-5;

FIGS. 12a-12b are coronal views of different cathode-anode electrodearrangements that can rostral-caudally expand a medial-lateralelectrical field relative to spinal cord tissue using theneurostimulation paddle lead of FIG. 5;

FIGS. 13a-13b are cross-sectional views of a cathode-anode electrodearrangement that can electrically modify the cathode-anode spacing usingany of the neurostimulation paddle leads of FIGS. 3-5;

FIGS. 14a-14b are cross-sectional views of a cathode-anode electrodearrangement that can create different medial-lateral electrical fieldson both sides of the midline of spinal cord tissue using any of theneurostimulation paddle leads of FIGS. 3-5; and

FIG. 15 is a coronal view of different electrode arrangements that canbe used to gradually shifting the locus of a stimulation region from amedial position to a left lateral position.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Turning first to FIG. 2, an exemplary spinal cord stimulation (SCS)system 10 generally comprises an implantable stimulation lead 12, animplantable pulse generator (IPG) 14 (or alternatively RFreceiver-stimulator), an external remote control RC 16, a Clinician'sProgrammer (CP) 18, an External Trial Stimulator (ETS) 20, and anexternal charger 22.

The IPG 14 is physically connected via a lead extension 24 to thestimulation lead 12, which carries a plurality of electrodes 26 arrangedin an array. As will be described in further detail below, the IPG 14includes pulse generation circuitry that delivers electrical stimulationenergy in the form of a pulsed electrical waveform (i.e., a temporalseries of electrical pulses) to the electrode array 26 in accordancewith a set of stimulation parameters.

The ETS 20, which has similar pulse generation circuitry as the IPG 14,also provides electrical stimulation energy to the electrode array 26 inaccordance with a set of stimulation parameters. The major differencebetween the ETS 20 and the IPG 14 is that the ETS 20 is anon-implantable device that is used on a trial basis after thestimulation lead 12 has been implanted and prior to implantation of theIPG 14, to test the effectiveness of the stimulation that is to beprovided.

The RC 16 may be used to telemetrically control the ETS 20 via abi-directional RF communications link 32. Once the IPG 14 andstimulation lead 12 are implanted, the RC 16 may be used totelemetrically control the IPG 14 via a bi-directional RF communicationslink 34. Such control allows the IPG 14 to be turned on or off and to beprogrammed with different stimulation programs after implantation. Oncethe IPG 14 has been programmed, and its power source has been charged orotherwise replenished, the IPG 14 may function as programmed without theRC 16 being present.

The CP 18 provides clinician detailed stimulation parameters forprogramming the IPG 14 and ETS 20 in the operating room and in follow-upsessions. The CP 18 may perform this function by indirectlycommunicating with the IPG 14 or ETS 20, through the RC 16, via an IRcommunications link 36. Alternatively, the CP 18 may directlycommunicate with the IPG 14 or ETS 20 via an RF communications link (notshown). The external charger 22 is a portable device used totranscutaneously charge the IPG 14 via an inductive link 38.

For purposes of brevity, the details of the RC 16, CP 18, ETS 20, andexternal charger 22 will not be described herein. Details of exemplaryembodiments of these devices are disclosed in U.S. Pat. No. 6,895,280,which is expressly incorporated herein by reference.

Referring further to FIG. 3, the IPG 14 comprises an outer case 40 forhousing the electronic and other components (described in further detailbelow), and a connector 42 in which the proximal end of the stimulationlead 12 mates in a manner that electrically couples the electrodes 26 tothe electronics within the outer case 40. The outer case 40 is composedof an electrically conductive, biocompatible material, such as titanium,and forms a hermetically sealed compartment wherein the internalelectronics are protected from the body tissue and fluids. In somecases, the outer case 40 serves as an electrode.

As will be described in further detail below, the IPG 14 includes pulsegeneration circuitry that provides electrical conditioning andstimulation energy to the electrodes 26 in accordance with a set ofparameters. Such parameters may comprise electrode combinations, whichdefine the electrodes that are activated as anodes (positive), cathodes(negative), and turned off (zero), and electrical pulse parameters,which define the pulse amplitude (measured in milliamps or voltsdepending on whether the IPG 14 supplies constant current or constantvoltage to the electrodes), pulse duration (measured in microseconds),and pulse rate (measured in pulses per second).

With respect to the pulse patterns provided during operation of the SCSsystem 10, electrodes that are selected to transmit or receiveelectrical energy are referred to herein as “activated,” whileelectrodes that are not selected to transmit or receive electricalenergy are referred to herein as “non-activated.” Electrical energydelivery will occur between two (or more) electrodes, one of which maybe the IPG case, so that the electrical current has a path from theenergy source contained within the IPG case to the tissue and a sinkpath from the tissue to the energy source contained within the case.Electrical energy may be transmitted to the tissue in a monopolar ormultipolar (e.g., bipolar, tripolar, etc.) fashion.

Monopolar delivery occurs when a selected one or more of the leadelectrodes is activated along with the case of the IPG 14, so thatelectrical energy is transmitted between the selected electrode andcase. Monopolar delivery may also occur when one or more of the leadelectrodes are activated along with a large group of lead electrodeslocated remotely from the one more lead electrodes so as to create amonopolar effect; that is, electrical energy is conveyed from the one ormore lead electrodes in a relatively isotropic manner. Multipolardelivery occurs when two or more of the lead electrodes are activated asanode and cathode, so that electrical energy is transmitted between theselected electrodes.

As best illustrated in FIG. 3, the stimulation lead 12 takes the form ofa surgical paddle lead that comprises an elongated body 44 having aproximal end 46 and a distal end 48, and a paddle-shaped membrane 50formed at the distal end 48 of the lead body 44. In an alternativeembodiment, the stimulation lead 12 may include multiple elongatedbodies, in which case, the paddle-shaped membrane 50 may be formed atthe distal ends of the elongated bodies. The lead body 44 may, e.g.,have a diameter within the range of 0.03 inches to 0.07 inches and alength within the range of 30 cm to 90 cm for spinal cord stimulationapplications. Each lead body 44 may be composed of a suitableelectrically insulative material, such as, a polymer (e.g., polyurethaneor silicone), and may be extruded from as a unibody construction. Thepaddle-shaped membrane 50 is composed of an electrically insulativematerial, such as silicone.

The stimulation lead 12 further comprises a plurality of terminals (notshown) mounted to the proximal end 46 of the lead body 44 and theplurality of electrodes 26 disposed on one side of the exterior surfaceof the paddle-shaped membrane 50 in a two-dimensional arrangement.Further details regarding the construction and method of manufacture ofpaddle leads are disclosed in U.S. patent application Ser. No.11/319,291, entitled “Stimulator Leads and Methods for LeadFabrication,” the disclosure of which is expressly incorporated hereinby reference.

Significant to some of the present invention, the electrodes 26 arearranged in four columns along the longitudinal axis of the stimulationlead 12. In particular, the electrodes 26 are arranged in two innercolumns of electrodes 26′ that are immediately adjacent to each other,and two outer columns of electrodes 26″ that flank and are immediatelyadjacent the respective inner electrode columns 26′. For the purposes ofthis specification, two electrode columns are immediately adjacent toeach other if no electrode column is disposed between the respectiveelectrode columns. As will be described in further detail below, the useof four or more electrode columns allows the locus of stimulation energyto be adjusted in the transverse direction, as well as provides theoption of electronically selecting wider or narrower cathode-anodespacings. Each of the electrodes 26 is composed of an electricallyconductive, non-corrosive, material, such as, e.g., platinum, titanium,stainless steel, or alloys thereof.

The stimulation lead 12 also includes a plurality of electricalconductors (not shown) extending through the lead body 44 and connectedbetween the respective terminals (not shown) and electrodes 26 usingsuitable means, such as welding, thereby electrically coupling theproximally-located terminals with the distally-located electrodes 26. Inthe case where the stimulation lead 12 includes multiple elongatedbodies, the proximally-located terminals on each lead body will beelectrically coupled to a specific column of electrodes 26 located onthe paddle-shaped membrane 50. In alternative embodiments, theelectrodes 26 may be arranged in more than four columns. For example, iffive columns are used, there may be three inner electrode columns, andtwo outer electrode columns flanking the three inner electrode columns.

Although the stimulation lead 12 is shown as having sixteen electrodes26, the number of electrodes may be any number suitable for theapplication in which the stimulation lead 12 is intended to be used(e.g., four, eight, twelve, twenty, etc.), as long as there are at leastfour electrodes extending along the paddle-shaped membrane 50 in adirection transverse to the longitudinal axis of the stimulation lead12.

In the embodiment illustrated in FIG. 3, each of the electrodes 26 has arectangular shape, with the longest dimension of the electrode beingoriented along the longitudinal axis of the stimulation lead 12, and theshortest dimension of the electrode being oriented transverse to thelongitudinal axis of the stimulation lead 12. In this manner, theelectrodes 26 may be more easily arranged on the paddle-shaped membrane50, which has a similar aspect ratio as the electrodes 26.Alternatively, the electrodes 26 may be square, circular, oblong,elliptical, or circular. Furthermore, the electrodes 26 may havenon-uniform shapes (e.g., some may be rectangular, and others may beelliptical). In the illustrated embodiment, each electrode may have alongitudinal dimension in the range of 1.0-4.0 mm, and a transversedimension in the range of 0.5-2.5 mm. Immediately adjacent electrodecolumns may have a transverse spacing 52 (as measured between thecenters of the columns) in the range of 1.5-4.0 mm, and immediatelyadjacent electrodes in each column may have a longitudinal spacing 54(as measured between the centers of the electrodes) in the range of1.5-4.0 mm. Preferably, a transverse spacing 56 between the outerelectrode columns (as measured between the centers of the columns) is inthe range of 4.5-11.0 mm. Ultimately, the transverse and longitudinalspacings between immediately adjacent electrodes should be small enough,so that activation of electrodes will have a spatially combined effect.

Although the electrode columns of the stimulation paddle lead 12illustrated in FIG. 3 are shown as having uniform transverse spacings,it may be advantageous to vary the transverse spacings between theelectrode columns. For example, referring to FIG. 4, the innertransverse spacing 52 a between the inner electrode columns 26′ may beless than the outer transverse spacings 52 b between each innerelectrode column 26′ and the respective adjacent outer electrode column26″. Preferably, the ratio between the inner spacing and the outerspacings is within the range of 0.10-0.75. As will be described infurther detail below, this configuration provides an optimum compromisebetween selectivity of the locus of the stimulation energy and theavoidance of inadvertent stimulation of dorsal root (DR) fibers.Alternatively, if stimulation of the DR fibers is desired, the innertransverse spacing 52 a between the inner electrode columns 26′ may bemore than the outer transverse spacings 52 b between each innerelectrode column 26′ and the respective adjacent outer electrode column26″.

Although the electrode columns of the stimulation paddle lead 12illustrated in FIG. 2 are shown as being longitudinally aligned witheach other (i.e., they are not offset from each other in thelongitudinal direction), it may be advantageous to offset some of theelectrode columns from each other. For example, referring to FIG. 5, theinner electrode columns 26′ are longitudinally offset from the otherouter electrode columns 26″ (i.e., the electrode columns 26′ are not atthe same longitudinal level). As there shown, only three electrodes 26are disposed within each inner electrode column 26′. In the embodimentillustrated in FIG. 5, each electrode in the inner electrode columns 26′is equi-distant to the two immediately adjacent electrodes in therespective outer electrode column 26″ (i.e., the outer electrode justabove and the outer electrode just below the respective innerelectrode). As will be described in further detail below, longitudinallyoffsetting the electrode columns in this manner provides for a moreefficient use of the stimulation energy to stimulate dorsal column (DC)fibers while preventing the inadvertent stimulation of dorsal root (DR)fibers.

Turning next to FIG. 6, one exemplary embodiment of the IPG 14 (andalternatively the ETS 20) will now be described. The IPG 14 includesstimulation output circuitry 60 configured for generating electricalstimulation energy in accordance with a defined pulsed waveform having aspecified pulse amplitude, pulse rate, and pulse duration under controlof control logic circuitry 62 over data bus 64. Control of the pulserate and pulse duration of the electrical waveform is facilitated bytimer logic circuitry 66, which may have a suitable resolution, e.g., 10μs. The stimulation energy generated by the stimulation output circuitry60 is output via capacitors C1-C16 to electrical terminals 68corresponding to electrodes E1-E16.

In the illustrated embodiment, the stimulation output circuitry 60comprises a plurality m independent current source pairs 70 capable ofsupplying stimulation energy to the electrical terminals 68 at aspecified and known amperage. One current source 72 of each pair 70functions as a positive (+) or anodic current source, while the othercurrent source 74 of each pair 70 functions as a negative (−) orcathodic current source. The outputs of the anodic current source 72 andthe cathodic current source 74 of each pair 70 are connected to a commonnode 76. The stimulation output circuitry 60 further comprises a lowimpedance switching matrix 78 through which the common node 76 of eachcurrent source pair 70 is connected to any of the electrical terminals68 via the capacitors C1-C16.

Thus, for example, it is possible to program the first anodic currentsource 72 (+I1) to produce a pulse having a peak amplitude of +4 mA (ata specified rate and for a specified duration), and to synchronouslyprogram the second cathodic current source 74 (−l2) to similarly producea pulse having a peak amplitude of −4 mA (at the same rate and pulseduration), and then connect the node 76 of the anodic current source 72(+I1) to the electrical terminal 68 corresponding to electrode E3, andconnect the node 76 of the cathodic current source 74 (−I2) to theelectrical terminal 68 corresponding to electrode E1.

Hence, it is seen that each of the programmable electrical terminals 68can be programmed to have a positive (sourcing current), a negative(sinking current), or off (no current) polarity. Further, the amplitudeof the current pulse being sourced or sunk from a given electricalterminal 68 may be programmed to one of several discrete levels. In oneembodiment, the current through each electrical terminal 68 can beindividually set from 0 to ±10 mA in steps of 100 μA, within the outputvoltage/current requirements of the IPG 14. Additionally, in oneembodiment, the total current output by a group of electrical terminals68 can be up to ±20 mA (distributed among the electrodes included in thegroup). Moreover, it is seen that each of the electrical terminals 68can operate in a multipolar mode, e.g., where two or more electricalterminals are grouped to source/sink current at the same time.Alternatively, each of the electrical terminals 68 can operate in amonopolar mode where, e.g., the electrical terminals 68 are configuredas cathodes (negative), and case of the IPG 14 is configured as an anode(positive).

It can be appreciated that an electrical terminal 68 may be assigned anamplitude and included with any of up to k possible groups, where k isan integer corresponding to the number of timing channels, and in oneembodiment, is equal to 4, and with each timing channel k having adefined pulse amplitude, pulse duration, and pulse rate. Other timingchannels may be realized in a similar manner. Thus, each channelidentifies which electrical terminals 68 (and thus electrodes) areselected to synchronously source or sink current, the pulse amplitude ateach of these electrical terminals, and the pulse duration, and pulserate.

The IPG 14 further comprises monitoring circuitry 80 for monitoring thestatus of various nodes or other points 82 throughout the IPG 14, e.g.,power supply voltages, temperature, battery voltage, and the like. Themonitoring circuitry 80 is also configured for measuring electrical dataat the electrodes 26 (e.g., electrode impedance and/or electrode fieldpotential) necessary to determine whether each of the electrodes 26 isfunctioning properly and is properly coupled to the IPG 14. In caseswhere the electrode array 12 is used to sense physiological information,the monitoring circuitry 80 may also have the appropriate circuitry(e.g., an analog/digital converter) for converting the physiologicalinformation sensed by the electrodes 26 into a form that can besubsequently analyzed. The physiological information at the electrodes26 may be measured using any one of a variety of means, but preferablyis made independent of the electrical stimulation pulses, as describedin U.S. Pat. Nos. 6,516,227 and 6,993,384, which are expresslyincorporated herein by reference.

The IPG 14 further comprises processing circuitry in the form of amicrocontroller (μC) 84 that controls the control logic circuitry 62over data bus 86, and obtains status data, and optionally physiologicalinformation, from the monitoring circuitry 80 via data bus 88. The IPG14 additionally controls the timer logic circuitry 66. The IPG 14further comprises memory 90 and an oscillator and clock circuit 92coupled to the microcontroller 84. Thus, the microcontroller 84, incombination with the memory 90 and oscillator and clock circuit 92,comprise a microprocessor system that carries out functions inaccordance with a suitable program stored in the memory 90.Alternatively, for some applications, the functions provided by themicroprocessor system may be carried out by a suitable state machine.

The microcontroller 84 generates the necessary control and statussignals, which allow the microcontroller 84 to control the operation ofthe IPG 14 in accordance with the operating program and stimulationparameters stored in the memory 90. In controlling the operation of theIPG 14, the microcontroller 84 is able to individually generate stimuluspulses at the electrodes 26 using the stimulation output circuitry 60,in combination with the control logic circuitry 62 and timer logiccircuitry 66, thereby allowing each electrode 26 to be paired or groupedwith other electrodes 26, including the monopolar case electrode, and tocontrol and modify the polarity, pulse amplitude, pulse rate, pulseduration, and channel through which the current stimulus pulses areprovided.

In the case wherein the IPG 14 processes physiological information(either sensed at the electrodes 26 via the monitoring circuitry 80 orsensed using a separate monitor), the algorithm used to electronicallydisplace the locus of the stimulation region based on the sensedphysiological information may be stored in the memory 90 for executionby the microcontroller 84 to appropriately control the stimulationoutput circuitry 60 via adjustment of the stimulation parameters. Inthis case, the microcontroller 84 will determine the stimulationparameters, including the electrode combination and individualamplitudes of the electrical energy at the electrodes 26, necessary toelectronically displace the locus of the stimulation region in anoptimum or otherwise more effective manner, and control the stimulationoutput circuitry 60 in accordance with these stimulation parameters.

The IPG 14 further comprises an alternating current (AC) receiving coil94 for receiving programming data (e.g., the operating program and/orstimulation parameters) from the RC 16 in an appropriate modulatedcarrier signal, and charging and forward telemetry circuitry 96 fordemodulating the carrier signal it receives through the AC receivingcoil 94 to recover the programming data, which programming data is thenstored within the memory 90, or within other memory elements (not shown)distributed throughout the IPG 14.

The IPG 14 further comprises back telemetry circuitry 98 and analternating current (AC) transmission coil 100 for sending informationaldata sensed through the monitoring circuitry 80 to the RC 16. The backtelemetry features of the IPG 14 also allow its status to be checked.For example, any changes made to the stimulation parameters areconfirmed through back telemetry, thereby assuring that such changeshave been correctly received and implemented within the IPG 14.Moreover, upon interrogation by the RC 16, all programmable settingsstored within the IPG 14 may be uploaded to the RC 16.

The IPG 14 further comprises a rechargeable power source 102 and powercircuits 104 for providing the operating power to the IPG 14. Therechargeable power source 102 may, e.g., comprise a lithium-ion orlithium-ion polymer battery. The rechargeable battery 102 provides anunregulated voltage to the power circuits 104. The power circuits 104,in turn, generate the various voltages 106, some of which are regulatedand some of which are not, as needed by the various circuits locatedwithin the IPG 14. The rechargeable power source 102 is recharged usingrectified AC power (or DC power converted from AC power through othermeans, e.g., efficient AC-to-DC converter circuits, also known as“inverter circuits”) received by the AC receiving coil 104. To rechargethe power source 102, an external charger (not shown), which generatesthe AC magnetic field, is placed against, or otherwise adjacent, to thepatient's skin over the implanted IPG 14. The AC magnetic field emittedby the external charger induces AC currents in the AC receiving coil104. The charging and forward telemetry circuitry 96 rectifies the ACcurrent to produce DC current, which is used to charge the power source102. While the AC receiving coil 104 is described as being used for bothwirelessly receiving communications (e.g., programming and control data)and charging energy from the external device, it should be appreciatedthat the AC receiving coil 104 can be arranged as a dedicated chargingcoil, while another coil, such as coil 100, can be used forbi-directional telemetry.

As shown in FIG. 6, much of the circuitry included within the IPG 14 maybe realized on a single application specific integrated circuit (ASIC)108. This allows the overall size of the IPG 14 to be quite small, andreadily housed within a suitable hermetically-sealed case.Alternatively, most of the circuitry included within the IPG 14 may belocated on multiple digital and analog dies, as described in U.S. patentapplication Ser. No. 11/177,503, filed Jul. 8, 2005, which isincorporated herein by reference in its entirety. For example, aprocessor chip, such as an application specific integrated circuit(ASIC), can be provided to perform the processing functions withon-board software. An analog IC (AIC) can be provided to perform severaltasks necessary for the functionality of the IPG 14, including providingpower regulation, stimulus output, impedance measurement and monitoring.A digital IC (DigIC) may be provided to function as the primaryinterface between the processor IC and analog IC by controlling andchanging the stimulus levels and sequences of the current output by thestimulation circuitry in the analog IC when prompted by the processorIC.

It should be noted that the diagram of FIG. 6 is functional only, and isnot intended to be limiting. Those of skill in the art, given thedescriptions presented herein, should be able to readily fashionnumerous types of IPG circuits, or equivalent circuits, that carry outthe functions indicated and described. Additional details concerning theabove-described and other IPGs may be found in U.S. Pat. No. 6,516,227,U.S. Patent Publication No. 2003/0139781, and U.S. patent applicationSer. No. 11/138,632, entitled “Low Power Loss Current Digital-to-AnalogConverter Used in an Implantable Pulse Generator,” which are expresslyincorporated herein by reference. It should be noted that rather than anIPG, the SCS system 10 may alternatively utilize an implantablereceiver-stimulator (not shown) connected to the stimulation lead 12. Inthis case, the power source, e.g., a battery, for powering the implantedreceiver, as well as control circuitry to command thereceiver-stimulator, will be contained in an external controllerinductively coupled to the receiver-stimulator via an electromagneticlink. Data/power signals are transcutaneously coupled from acable-connected transmission coil placed over the implantedreceiver-stimulator. The implanted receiver-stimulator receives thesignal and generates the stimulation in accordance with the controlsignals.

Referring to FIG. 7, the stimulation lead 12 is implanted within thespinal column 110 of a patient 108. The preferred placement of thestimulation lead 12 is adjacent, i.e., in the epidural space above thespinal cord area to be stimulated. Due to the lack of space near thelocation where the stimulation lead 12 exit the spinal column 110, theIPG 14 is generally implanted in a surgically-made pocket either in theabdomen or above the buttocks. The IPG 14 may, of course, also beimplanted in other locations of the patient's body. The lead extension24 facilitates locating the IPG 14 away from the exit point of thestimulation lead 12. After implantation, the IPG 14 is used to providethe therapeutic stimulation under control of the patient.

Referring now to FIG. 8, the stimulation lead 12 may be implanted withinthe patient, such that the electrodes 26 are disposed adjacent thespinal cord, with the inner electrode columns 26′ disposed directly overthe dorsal column (DC) fibers, and the outer electrode columns 26″disposed directly over the dorsal root (DR) fibers. Notably, thestimulation lead 12 illustrated in FIG. 5 is used, so that therelatively small spacing between the inner electrode columns 26′maintains them away from the DR fibers to prevent the inadvertentstimulation of the DR fibers. The relatively large spacing between theinner electrode columns 26′ and the respective outer electrode columns26″ places the outer electrode columns 26″ directly over the DR fibersto guard against overstimulation of the DR fibers.

In particular, the IPG 14 may configure at least one of the electrodesin the inner electrode columns 26′ as a cathode, and at least one of theelectrodes in the outer electrode columns 26″ as an anode, such thatelectrical stimulation energy conveyed from the IPG 14 between thecathode(s) and anode(s) creates a medial-lateral electrical field thatstimulates the DC fibers, while preventing stimulation of the DR fibers.That is, electrical energy originating from the cathode(s) stimulatesthe DC fibers, while the anode(s) “push” the electrical stimulationenergy away from the DR fibers. For the purposes of this specification,a “medial-lateral field” means that the strongest field components areoriented approximately parallel to the medial-lateral axis, as opposedto a “rostral-caudal field,” which means that the strongest fieldcomponents are oriented approximately parallel to the rostral-caudalaxis.

In the electrode arrangement illustrated in FIG. 8, the IPG 14configures a single row of inner electrodes 26′ (i.e., two immediatelyadjacent and longitudinally aligned electrodes of the respective innerelectrode columns 26′) as cathodes (cathodes shown with “c”), whileconfiguring the two rows of outer electrodes 26″ that flank the cathodes(i.e., the two immediately adjacent electrodes of the left outerelectrode column 26″ that longitudinally flank the cathode in the leftinner electrode column 26′ and the two immediately adjacent electrodesof the right outer electrode column 26″ that longitudinally flank thecathode in the right inner electrode column 26′) as anodes (anodes shownwith “a”). The IPG 14 further configures the cathodes to have afractionalized cathodic current of 50% each, and the anodes to have afractionalized anodic current of 25% each. Thus, in the case where themidline of the spinal cord is equi-distant between the cathodes, thelocus of the applied electrically energy will be located on the midlineof the spinal cord. Of course, if midline of the spinal cord is offsetfrom the cathodes, the locus of the applied electrical energy will belocated offset from the midline of the spinal cord, which may be desiredin some circumstances.

Notably, as compared to a cathode-anode configuration used with thestimulation lead 12 illustrated in FIG. 2 or 3 in which a single row ofelectrodes may be activated (two adjacent electrodes in the innerelectrode columns 26′ being configured as cathodes, and an adjacentelectrode in each of the outer electrode columns 26″ being configured asanodes), as illustrated in FIG. 9, it has been discovered that theparticular cathode-anode configuration illustrated in FIG. 8 provides arelatively efficient stimulation. Although each of the anodes in thecathode-anode configuration illustrated in FIG. 8 has a fractionalizedanodic current that is less than each of the anodes in the cathode-anodeconfiguration illustrated in FIG. 9, thereby providing somewhat less ofa guarding effect against overstimulation of the DR fibers, the cathodesin the cathode-anode configuration illustrated in FIG. 8 shunt lesscathodic current than the cathodes in the cathode-anode configurationillustrated in FIG. 9, because the anode is more distributed. Withappropriate geometries, the reduction in the guarding effect is smallerthan the improvement in the shunting effect, yielding improvedefficiency.

The electrodes in the arrangement illustrated in FIG. 8 can be variouslyconfigured to transversely adjust the locus of the stimulation energy,as well as to provide varying depths of electrical stimulation. In onemethod, immediately adjacent electrodes in the left and right innerelectrode columns 26′ may be configured as cathodes, and two adjacentelectrodes that longitudinally flank the cathodes in only one of theouter electrode columns 26″ may be configured as anodes. In this case,the anodes in one of the left or right outer electrode columns 26″ areused to “push” the locus of the stimulation energy towards the other ofthe left or right outer electrode columns 26″.

For example, in a cathode-anode configuration illustrated in FIG. 10a ,a single row of the electrodes in the inner electrode columns 26′ (twoimmediately adjacent and longitudinally aligned electrodes) areconfigured as cathodes (with a fractionalized cathodic current of 50%each), and two immediately adjacent and longitudinally flankingelectrodes in the left outer electrode column 26″ are configured asanodes (with a fractionalized anodic current of 50% each). In this case,the anodes “push” the locus of stimulation energy towards theinactivated outer electrode column 26″ (i.e., the right outer electrodecolumn 26″).

In another method, an electrode in only one of the left and right innerelectrode columns 26′ may be configured as a cathode, and two adjacentelectrodes that longitudinally flank the cathodes in only one or in bothof the outer electrode columns 26″ may be configured as anodes. In thesecases, the cathode in one of the left or right outer electrode columns26″ is used to shift the locus of the stimulation energy towards the oneleft or right outer electrode column 26″. If the electrodes in both ofthe outer electrode columns 26″ are configured as anodes, they will notserve to “push” the locus of stimulation energy, but will merely serveto guard both the left and right dorsal root (DR) fibers fromstimulation.

For example, in a cathode-anode configuration illustrated in FIG. 10b ,a single electrode in the right inner electrode column 26′ is configuredas a cathode, and two immediately adjacent and longitudinally flankingelectrodes in the left outer electrode column 26″ and in the right outerelectrode column 26″ are configured as anodes (with a fractionalizedanodic current of 25% each). In this case, the single cathode shifts thelocus of stimulation energy to the right of the midline of the dorsalcolumn (DC), and the anodes guard the DR fibers against stimulation.

If the electrodes in only one of the outer electrode columns 26″ areconfigured as anodes, they will serve to “push” the locus of stimulationenergy towards the other outer electrode column 26″. For example, in acathode-anode configuration illustrated in FIG. 10c , a single electrodein the right inner electrode column 26′ is configured as a cathode, andtwo immediately adjacent and longitudinally flanking electrodes in theleft outer electrode column 26″ are configured as anodes (with afractionalized anodic current of 50% each). In this case, the singlecathode moves the locus of stimulation energy to the right of themidline of the spinal cord, and the anodes further “push” the locus ofstimulation energy towards the inactivated right outer electrode column26″.

In the case where the electrodes in both of the outer electrode columns26″ are configured as anodes, the electrodes in the outer electrodecolumn 26″ that are immediately adjacent to the only inner electrodecolumn 26″ in which the electrode is configured as a cathode can have afractionalized current that is greater than the electrodes in the otherouter electrode column 26″ to further enhance their guarding effect withrespect to the DR fibers that are closest to the activated cathode.

For example, in a cathode-anode configuration illustrated in FIG. 10d ,a single electrode in the right inner electrode column 26′ is configuredas a cathode, and two immediately adjacent and longitudinally flankingelectrodes in the left outer electrode column 26″ and in the right outerelectrode column 26″ are configured as anodes (with a fractionalizedanodic current of 10% for the left anodes, and a fractionalized anodiccurrent of 40% for each of the right anodes). In this case, the singlecathode moves the locus of stimulation energy to the right of themidline of the DC, the anodes in the left outer electrode column 26″guard the left DR fibers from stimulation, and the anodes in the rightouter electrode column 26″ guard the right DR fibers from stimulation.Notably, the fractionalized current for the anodes in the rightelectrode outer column 26″ is greater than the fractionalized currentfor the anodes in the left electrode outer column 26″, since theactivated cathode is closer to the right DR fibers. Based on a modeledcathode-anode configuration that assumes a dCSF of 2.0 mm and a currentoutput of 1.70 mA, a total of 1184 DC fibers were stimulated in thecathode-anode model of FIG. 9 d.

The locus and size of the medial-lateral electrical field generated bythe electrodes in the arrangement illustrated in FIG. 8 can befine-tuned through the use of current steering. In doing this, the IPG14 may first configure at least two electrodes in the inner electrodecolumns 26′ as cathodes, and at least one electrode of each of the outerelectrode columns 26″ as anodes. The IPG 14 may then convey electricalenergy between the cathodes and the anodes to create a medial-lateralelectrical field that stimulates the spinal cord tissue (preferablystimulating the DC fibers without over-stimulating the DR fibers), andincrementally shifting cathodic current between the cathodes to modifythe medial-lateral electrical field. Notably, for the purposes of thisspecification, current is incrementally shifted between two electrodesif the current is gradually shifted from one electrode to the otherelectrode over a few steps. Preferably, cathodic current is shiftedbetween cathodes in increments equal to 10% or less. Navigation tables,such as those described in U.S. patent application Ser. No. 11/557,477,entitled “System and Method for Uniformly Displacing a Region of NeuralStimulation,” now issued as U.S. Pat. No. 7,742,819 and U.S. patentapplication Ser. No. 12/614,942, entitled “System and Method forDetermining Appropriate Steering Tables for Distributing StimulationEnergy Among Multiple Neurostimulation Electrodes,” which are expresslyincorporated herein by reference, can be used to steer current betweenelectrodes.

In one embodiment, the electrodes that are configured as cathodes arelongitudinally aligned relative to each other (or rostral-caudallyaligned relative to each other when implanted within the patient), suchthat incremental shifting of the current between the cathodes spatiallyshifts the medial-lateral electrical field transversely relative to thelongitudinal axis (i.e., relative to the DC fibers of the spinal cordtissue).

For example, initially assuming an equal distribution of cathodiccurrent between the two cathodes (50% in each)(FIG. 11a ), 10% of thecathodic current can be shifted from the left cathode to the rightcathode (40% in left cathode and 60% in the right cathode)(FIG. 11b ),thereby shifting the locus of the medial-lateral electrical field fromthe midline of the spinal cord tissue to a location offset to the rightof the midline. 10% of the cathodic current can further be shifted fromthe left cathode to the right cathode (30% in left cathode and 70% inthe right cathode) (FIG. 11c ). The cathodic current can be shifted fromthe left cathode to the right cathode a number of times until none ofthe cathodic current flows through the left cathode and all of thecathodic current flows through the right cathode.

In a similar manner, initially assuming an equal distribution ofcathodic current between the two cathodes (50% fractionalized current ineach)(FIG. 11a ), 10% of the cathodic current can be shifted from theright cathode to the left cathode (60% fractionalized current in leftcathode and 40% fractionalized current in the right cathode)(FIG. 11d ),thereby shifting the locus of the medial-lateral electrical field fromthe midline of the spinal cord tissue to a location offset to the leftof the midline. 10% of the cathodic current can further be shifted fromthe right cathode to the left cathode (70% fractionalized current inleft cathode and 30% fractionalized current in the right cathode) (FIG.11e ). The cathodic current can be shifted from the right cathode to theleft cathode a number of times until none of the cathodic current flowsthrough the right cathode and all of the cathodic current flows throughthe left cathode.

In another embodiment, the electrodes that are configured as cathodesare longitudinally offset from each other (i.e., rostral-caudally offsetfrom each other), such that incremental shifting of the cathodic currentbetween the cathodes expands the medial-lateral electrical field alongthe longitudinal axis (i.e., rostral-caudally expands the medial-lateralelectrical field). This can be accomplished, e.g., when the dCSF isrelatively large, thereby reducing the collective electrode impedance.When the dCSF is large enough, increasing the distribution of thecurrent longitudinally among more electrodes will make little differencein the field at the spinal cord, but has improved collective impedanceproperties.

For example, referring to FIG. 12a , it is first assumed that fouradjacent electrodes are configured with cathodes, with the bottom leftcathode having a fractionalized current of 20%, the middle left cathodehaving a fractionalized current of 60%, the bottom right cathode havinga fractionalized current of 10%, and the middle right cathode having afractionalized current of 10%, and three anodes in each of the outerelectrode columns are configured as anodes, with the bottom left anodehaving a fractionalized current of 10%, the middle left anode having afractionalized current of 20%, the top left anode having afractionalized current of 20%, the bottom right anode having afractionalized current of 10%, the middle right anode having afractionalized current of 20%, and the top right anode having afractionalized current of 20%. Based on this configuration, the locus ofthe medial-lateral electrical field will be left of the midline andnearer the top cathodes.

As shown in FIG. 12b , the medial-lateral electrical field can beexpanded in the rostral direction by configuring an upper row ofelectrodes as cathodes, and incrementally shifting some of the currentfrom the lower four cathodes to the upper two cathodes. In this case,some of the cathodic current is shifted to the upper cathodes, such thateach of the left cathodes has a fractionalized current of 20%, thebottom right cathode has a fractionalized current of 10%, the middleright cathode has a fractionalized current of 20%, and the top rightcathode has a fractionalized current of 10%. Anodic current is alsoshifted to the upper anodes, such that each of the anodes has afractionalized current of 12.5%.

The electrode arrangement illustrated in FIG. 8 can also be reconfiguredto selectively change the effective cathode-anode spacing. In doingthis, the IPG 14 may configure a first one of electrodes in the innerelectrode column 26′ as a cathode, a second one of the electrodes in theinner electrode column 26′ as one of a cathode and an anode, and two ofthe flanking electrodes in the respective outer electrode columns 26″ asanodes. The IPG 14 may then convey electrical energy between thecathodes and the anodes to create a medial-lateral electrical field thatstimulates the spinal cord tissue. The IPG 14 may then reconfigure thesecond inner electrodes as the other of the cathode and the anode, andreconveying electrical energy between the cathodes and the anodes tocreate a medial-lateral electrical field that stimulates the spinal cordtissue.

For example, referring to FIG. 13a , both of the inner electrodes areconfigured as cathodes, with the left cathode having a fractionalizedcurrent of 20%, and the right cathode having a fractionalized current of80%, and both of the outer electrodes are configured as anodes, with theleft anode having a fractionalized current of 70%, and the right anodehaving a fractionized current of 30%. In this case, the effectivecathode-anode spacing will be relatively large, so that, in the case ofa large dCSF, the interelectrode impedance will be reduced, therebyincreasing the size of the electrical field. Referring to FIG. 13b , theleft inner electrode is reconfigured as an anode, with the inner anodehaving a fractionalized current of 40%, the right cathode having afractionalized current of 100%, the left outer anode having afractionalized current of 10%, and the right outer anode having afractionalized current of 50%. In this case, the effective cathode-anodespacing is relatively small, so that, in the case of a small dCSF, themedial-lateral electrical field can be more finely tuned.

The electrode arrangement illustrated in FIG. 8 can also be forstimulating the DC fibers on both lateral sides to the midline withoutstimulating DC fibers along the midline. In particular, the IPG 14 mayconvey electrical energy between the electrodes to create amedial-lateral electrical field having a locus on one lateral side ofthe midline of the spinal cord tissue (e.g., by configuring only a firstone of the inner electrodes 26′ as a cathode or configuring a first oneof the inner electrodes 26′ to have more cathodic current than a secondone of the inner electrodes 26′), and conveying electrical energybetween the electrodes to create a medial-lateral electrical fieldhaving a locus on the other lateral side of the midline of the spinalcord tissue (e.g., by configuring only a second one of the innerelectrodes 26′ as a cathode or configuring a second one of the innerelectrodes 26′ to have more cathodic current than a first one of theinner electrodes 26′). The IPG 14 may utilize two channels to repeatedlymove the locus on of the medial-lateral electrical field back and forthbetween the left and right sides of the midline.

For example, referring to FIG. 14a , the right inner electrode isconfigured as a cathode, and the outer left and right flankingelectrodes are configured as anodes, with the right cathode having allof the cathodic current, the left anode having a fractionalized anodiccurrent of 20%, and the right anode having a fractionalized anodiccurrent of 80%. In this case, the locus of the medial-lateral electricalfield will be to the right of the midline. As a result, only the DCfibers on the right side of the midline will be stimulated. Referring toFIG. 14b , the left inner electrode is configured as a cathode, and theouter flanking electrodes are configured as anodes, with the left innerelectrode having all of the cathodic current, the left outer electrodehaving a fractionalized anodic current of 80%, and the right outerelectrode having a fractionalized anodic current of 20%. In this case,the locus of the medial-lateral electrical field will be to the left ofthe midline. As a result, only the DC fibers on the left side of themidline will be stimulated. Since some have hypothesized that low-backfibers are lateral to the midline, this system and method may beparticularly useful for low-back fiber stimulation.

With reference to FIG. 15, a technique for gradually shifting the locusof a stimulation region from a medial position to a left lateralposition (i.e., from right to left) will now be described. In eachelectrode configuration, electrodes activated as cathodes are shown withdark solid black lines, electrodes activated as anodes are shown withgrey solid lines, and non-activated electrodes are shown with dashedlines. The stimulation region generated by each electrode configurationis shown as an oval with dotted lines. This technique is especiallyuseful when the paddle lead has migrated, and in this case, migratedfrom a medial position to a right lateral position, in which case, thelocus of the stimulation region must be shifted back to the leftrelative to the paddle lead to maintain the same therapeutic effect.

Notably, when stimulating DC fibers, it is desirable that the ratio ofthe dorsal root (DR) threshold over the dorsal column (DC) threshold beas high as possible in order to minimize stimulation of the DR fibers.However, when shifting the stimulation region from a medial position toa lateral position, the DR/DC ratio will inevitably decrease. Tominimize this adverse effect, it is preferably that the DR/DC ratio bedecreased as smoothly and as little as possible. In making thistransition as smoothly as possible, this technique combinesrostro-caudal current steering using a longitudinal tripole (i.e., twoanodes or anode groups with a cathode or cathode group) and quadrapole(i.e., three anodes or anode groups with a cathode or cathode group)with medio-lateral current steering.

For example, in the initial electrode configuration (a), two adjacentelectrodes in the inner electrode columns are configured as cathodes,with each cathode having a fractionalized current of 50%, and twoelectrodes in each of the outer electrode columns are configured asanodes with each anode having a fractionalized current of 25%. Thiselectrode configuration generates a medial-lateral electrical field thatcreates a stimulation region having its locus at the midline of thecathodes.

Next, the electrical current can be medio-laterally steered to one ofthe electrode configurations (b1), (b2), or (b3) to gradually displacethe locus of the stimulation region to the left.

In electrode configuration (b1), two adjacent electrodes in the innerelectrode columns are configured as cathodes, with the left cathodehaving a fractionalized current of 75% and the right cathode having afractionalized current of 25%, and two electrodes in each of the outerelectrode columns are configured as anodes with each anode having afractionalized current of 25%. This electrode configuration displacesthe locus of the stimulation region to the left of the midline of thecathodes via cathode steering.

In electrode configuration (b2), two adjacent electrodes in the innerelectrode columns are configured as cathodes, with the left cathodehaving a fractionalized current of 67% and the right cathode having afractionalized current of 33%, and two electrodes in each of the outerelectrode columns are configured as anodes, with each of the left twoanodes having a fractionalized current of 10%, and each of the right twoanodes having a fractionalized current of 40%. This electrodeconfiguration displaces the locus of the stimulation region to the leftof the midline of the cathodes via cathode/anode steering.

In electrode configuration (b3), two adjacent electrodes in the innerelectrode columns are configured as cathodes, with each of the cathodeshaving a fractionalized current of 50%, and two electrodes in only theright outer electrode columns are configured as anodes, with each of theright two anodes having a fractionalized current of 50%. This electrodeconfiguration displaces the locus of the stimulation region to the leftof the midline of the cathodes via anode steering.

Next, the electrical field can be medio-laterally steered from one ofthe electrode configurations (b1), (b2), or (b3) to electrodeconfiguration (c) to gradually displace the locus of the electricalfield to the left. In electrode configuration (c), two adjacentelectrodes in the left outer electrode columns are configured ascathodes, and two electrodes in the left outer electrode columnsimmediately rostro-caudally flanking the cathodes are configured asanodes. This electrode configuration generates a rostro-caudalelectrical field that creates a stimulation region having its locuscentered on the left electrode column between the cathodes.

Next, the rostro-caudal electrical field is gradually broadened byshifting the cathodic current from electrode configuration (c) toelectrode configuration (d), wherein the anodes are moved further out inthe rostral/caudal direction. This will smooth out the transition to thenext medio-lateral electrode configuration.

Then, electrode configuration (d) is gradually changed to electrodeconfiguration (e), wherein the two adjacent electrodes in the leftcolumn remain cathodes that have a fractionalized current of 50% each,and some of the current has been shifted from the two anodes in the leftcolumn to three anodes in the left inner column adjacent the cathodes,so that each of the anodes has a fractionalized current of 20%. Thiselectrode configuration results in a medio-lateral electrical field thatdisplaces the locus of the stimulation region slightly to the left ofthe cathodes.

Next, electrode configuration (e) is gradually changed to electrodeconfiguration (f) by shifting current in the two anodes rostro-caudallyflanking the center anode in the left inner column to the centerelectrode, wherein the two adjacent electrodes in the left column remaincathodes that have a fractionalized current of 50% each, and all of thecurrent in the flanking anodes in the left inner column has been shiftedto the center anode, so that the anodes in the left outer column eachhas a fractionalized current of 20%, and the cathode in the left innercolumn has a fractionalized current of 60%. This electrode configurationresults in a medio-lateral electrical field that further displaces thelocus of the stimulation region to the left of the cathodes.

Next, electrode configuration (f) is gradually changed to electrodeconfiguration (g) by shifting the current from the two anodes in theleft outer column to the electrodes in the left inner column thatrostro-caudally flank the center anode, wherein the two adjacentelectrodes in the left column remain cathodes that have a fractionalizedcurrent of 50% each, and all of the current in the flanking anodes inthe left outer column has been shifted to the flanking anodes in theleft inner column, so that the flanking anodes in the left outer columneach has a fractionalized current of 20%, and the cathode in the leftinner column has a fractionalized current of 60%. This electrodeconfiguration results in a medio-lateral electrical field that narrowsthe stimulation region and further displaces its locus to the left ofthe cathodes.

Lastly, electrode configuration (g) is gradually changed to electrodeconfiguration (h) by shifting the current from the three anodes in theleft inner column to the center electrode in the right inner column,wherein the two adjacent electrodes in the left column remain cathodeshaving a fractionalized current of 50% each, and the anode in the rightinner column has a fractionalized current of 100%. This electrodeconfiguration results in a medio-lateral electrical field that displacesthe locus of the stimulation region even further to the left of thecathodes.

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

What is claimed is:
 1. A neurostimulation paddle lead, comprising: anelongated lead body having a proximal end, a distal end, and alongitudinal axis; a plurality of terminals carried by the proximal endof the lead body; a paddle-shaped membrane disposed on the distal end ofthe lead body; and a plurality of electrodes arranged on an exteriorsurface of the paddle-shaped membrane in electrical communication withthe respective terminals, the plurality of electrodes comprising atleast four columns of electrodes, each column extending along thepaddle-shaped membrane in a longitudinal direction, the at least fourelectrode columns having at least two inner electrode columns and twoouter electrode columns flanking the at least two inner electrodecolumns, wherein at least one of the at least two inner electrodecolumns is offset from the outer electrode columns in the longitudinaldirection.